1. Field of the Invention
The invention relates to a process for removing cellulose ethers, especially cellulose ethers with plastic properties and/or a high flocculation point, from a cellulose ether suspension, in which the cellulose ethers are removed as the filter cake by pressure filtration, especially in a rotary pressure filter.
2. Description of Related Art
Cellulose ether is prepared by etherification of alkali cellulose with methyl chloride at temperatures of about 92.degree. to 110.degree. C., as is described in detail in Ullmanns Enzyklopadie der technischen Chemie (Ullmanns Encyclopedia of Industrial Chemistry), 4th Ed., vol. 9, p. 205. In process, following the etherification reaction, the reaction mixture formed is introduced into an agitator vessel containing deionized water at about 95.degree. C. In the agitator vessel, the sodium chloride formed during the reaction is dissolved out, and the residual alkali that is still present is neutralized. The suspension that leaves the agitator vessel must then be subjected to a separation in order to obtain the desired pure cellulose ether.
In known processes, this separation is performed almost exclusively on screen centrifuges. However, one drawback of such screen centrifuges is the high loss of useful product that occurs upon impact. In addition, screen centrifuges are not suitable for products with high flocculation points or plastic properties.
Therefore attempts have previously been made to perform the workup of cellulose ethers on rotary pressure filters. However, under the extreme process conditions used, namely comparatively high pressures and high temperatures, the customary textile filter coverings for pressure filters cause problems. For example, the alternating pressure stress causes stretching, material fatigue, and ripping out of the textile fabric, and in addition the fabric shows a strong tendency to stick. Furthermore, the fabric can no longer be cleaned even by intensive rinsing, so that it becomes unusable after only a short filtration time.